The new mineral magganasite, ideally CuFe3+3O(AsO4)3, was found in the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with sanidine, lammerite, paralammerite, johillerite, calciojohillerite, alarsite, hematite, tenorite, cassiterite, langbeinite, euchlorine, fedotovite and wulffite. Magganasite forms prismatic crystals up to 0.2 mm long and up to 0.04 mm thick typically assembled in clusters up to 1 mm across. It is translucent, golden- or red-brown to brownish-yellow, with strong vitreous to semi-metallic lustre. Dcalc is 4.708 g cm–3. In reflected light, magganasite is grey, weakly anisotropic. The reflectance values [Rmax–Rmin,% (λ, nm)] are: 13.1–13.1 (470), 12.5–11.9 (546), 12.2–11.9 (589), 11.9–11.8 (650). Chemical composition (wt.%, electron microprobe data) is: CuO 14.69, ZnO 0.06, Al2O3 0.49, Cr2O3 0.19, Fe2O3 30.62, TiO2 2.22, SiO2 0.21, P2O5 0.12, V2O5 0.10, As2O5 50.72, SO3 0.63, total 100.05. The empirical formula based on 13 O apfu is Cu1.22Al0.06Cr0.02Fe3+2.52Ti0.18(As2.91S0.05Si0.02P0.01V0.01)Σ3.00O13. Magganasite is triclinic, P$\bar 1$, a = 5.1813(7), b = 9.6427(11), c = 9.6834(11) Å, α = 82.066(10), β = 78.680(11), γ = 79.962(10)°, V = 464.41(10) Å3 and Z = 2. The strongest reflections of the PXRD pattern [d,Å(I)(hkl)] are: 3.761(100)(102, 120), 3.540(46)(022), 3.280(88)(1$\bar 1$2), 3.204(62)($\bar 1$20), 3.170(41)($\bar 1$02, 030), 2.989(51)($\bar 1$12), 2.889(65)(103, 130), and 2.510(64)(200). The crystal structure, solved from single-crystal XRD data (R = 0.0609), is unique. It is based on the polyhedral ribbons built by two zig-zag chains of edge-sharing Fe2- and Fe3-centred octahedra linked via dimers of edge-sharing Cu-centred distorted tetragonal pyramids. The neighbouring ribbons are linked via dimers of edge-sharing Fe1-centred octahedra. AsO4 tetrahedra reinforce the linkage between the ribbons. The mineral is named in honour of the Greek mineralogist and petrologist Prof. Dr. Andreas Magganas (born 1954).